In present day police, military, and other intelligence and security operations, increasing numbers of operations are conducted that involve entering and securing above-ground or below-ground structures. Personnel involved with the tactical assessment of adversarial underground facilities and aboveground buildings are put at risk if an adversary is located within the structure. In such a case, a person entering the structure may be attacked by the adversary, or personnel in and/or around the structure may be subject to a surprise attack. Accordingly, operational planning involving such structures generally requires an abundance of caution in order to protect the involved personnel. Due to the many unknown risk factors involved, such planning and operations often proceed at a relatively slow pace. Once these facilities are identified, the ability to quickly and accurately assess the structures would improve operation planning and significantly lower the risks to soldiers and security personnel.
There has been a great deal of work done to detect and localize underground facilities, however, there are currently few sensing devices that can characterize these structures. Ground, air and space-borne sensors may locate underground facilities using passive acoustic (listening), infrared and other observables. Once located, tactical personnel must determine the threats posed by these facilities. Several sensors exist for locating personnel hidden within structures but most are impractical for use in caves and tunnels. Similarly, there are few devices that can identify potentially hostile movement within urban buildings. “Through the wall” radar is currently in routine use by law enforcement and border security personnel to detect subjects on the other side of building walls and some cargo containers. However, the range of detection is limited. Like most electromagnetic waves, radar is severely attenuated by rock/soil or metal walls and would not penetrate far into underground structures or buildings. Passive acoustic sensors have been developed to listen for nearby human activity (or heartbeats) within structures, and these may be very helpful in determining if humans are present in caves and tunnels. However, these sensors do not provide any information about the underground structure or location of activity deep inside a structure. In the case of underground facilities, military analysts have concluded that “the sheer complexity of underground facilities, including their location, depth, configuration, and military functions, suggest that this problem will plague U. S. defense planners for years to come”. Thus, new sensing methods are needed for characterizing configuration and activity within tunnels and caves.
Similarly, homeland security and urban combat situations require sensors that detect activity within buildings and other structures. Law enforcement agencies, rescue teams and fire departments need better “see through the wall” technologies to locate people within these structures.